Chemical Physics Letters 513 (2011) 145–148

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Chemical Physics Letters

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The kinetics of electron transfer across the multi-point contact interfacethrough simplifying the complex structure in dye-sensitized solar cell

Weiqing Liu a,b, Dongxing Kou a, Linhua Hu a, Songyuan Dai a,⇑a Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, PR Chinab Key Laboratory of Nondestructive Testing of Ministry of Education and School of the Testing and Photoeletric Enginering, Nanchang Hangkong University, Nanchang, Jiangxi 330063,PR China

a r t i c l e i n f o

Article history:Received 27 June 2011In final form 22 July 2011Available online 28 July 2011

0009-2614/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.cplett.2011.07.070

⇑ Corresponding author.E-mail address: sydai@ippa.ac.cn (S. Dai).

a b s t r a c t

In the dye-sensitized solar cell, a complex interface including the conductive glass, nano-TiO2 porous filmand electrolyte, is formed at the conductive substrate of the anode. The impedance and kinetic informa-tion about the transfer of electrons at the conductive glass/nano-TiO2 film contact through electrochem-ical impedance spectroscopy are obtained in this Letter after shielding the influence of electrolyte. Resultsshow that the time constant for the electron transfer across the TCO/TiO2 interface is about 10�6 s and thecollection process of electrons is confined by the electron transport in nano-TiO2 film.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Dye-sensitized solar cell (DSC) is a complicated photo-electro-chemical system which has progressed greatly during the past dec-ades [1,2]. The working mechanism of DSC is not yet fullyunderstood and needs further experimental investigation. TypicalDSC is based on a sandwich structure, which consists of anode,electrolyte and cathode. Electrons generated by photo-exciteddye molecules are injected into the conduction band of the TiO2

and transported from the injection sites to the contact electrodeunder light irradiation. Finally, electrons are collected at the trans-parent conductive oxide (TCO) substrate/TiO2 interface and thenpass through the external circuit.

In the anode, the TiO2 particles are randomly deposited on theconductive substrate and the TiO2 film cannot completely coverTCO substrate [3]. Since the exposed TCO also contacts the electro-lyte, a complex structure (TCO/TiO2 + EL interface), made up ofTCO, TiO2 and electrolyte, will form at the conductive substrate ofthe anode. Due to the presence of the electrolyte, many propertiesof the TCO/TiO2 interface become complicated [4]. Usually SnO2:Fconducting glass is used as the TCO and this type of TCO showsmetallic properties. So it was suggested that the TCO/TiO2 interfaceis modeled as a metal/semiconductor contact [5]. There were dis-agreements about whether an accumulation layer or a barrier ex-isted at TCO/TiO2 interface [4–9]. The behaviors of the TCO/TiO2

interface may be influenced by the presence of the electrolyte[5,10–12]. In the dark, the potential distribution at the TCO/TiO2 + ELinterface depends on the interfacial potential difference at the con-ductive glass/electrolyte contact (TCO/EL interface) [7]. The process

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of light-induced electrons transfer across the TCO/TiO2 + EL inter-face is still not clear. At present, there are several theoretical models,such as the junction model [13], the thermal emission model [14,15],and the tunnel-junction model [8] that are trying to interpret theprocess of electrons transfer across the TCO/TiO2 interface, but ithas not been completely solved yet [8]. Until now, we could not findthe interfacial impedance of TCO/TiO2 in a typical electrochemicalimpedance spectroscopy (EIS) plot. Subsequently, the time constantof electron collection has not been obtained in DSC.

In our Letter, an insulating layer was electropolymerized on theexposed TCO to separate the electrolyte from the TCO/TiO2 + ELinterface which simplifies the complicated structure and leavesonly the TCO/TiO2 interface. By analyzing the obtained impedanceinformation we have figured out some basic problems about theTCO/TiO2 interface and evaluated the kinetic behavior of electrontransfer at the TCO/TiO2 interface in DSC.

2. Experimental section

The TiO2 paste was prepared from a colloidal dispersion ob-tained by hydrolysis of titanium tetraisopropoxide as describedelsewhere [1,16]. The TiO2 films about 14 lm in thickness were ob-tained by screen printing on the TCO (TEC-15, LOF) substrate, thensintering at 450 �C for 30 min in the air. The active area of DSC was0.5 cm � 0.5 cm = 0.25 cm2.

The photoelectrodes were immersed in an ethanol solution(0.5 mM) of dye N719[cis-dithiocyanate-N,N0-bis-(4-carboxylate-40-tetrabutylamonium-carboxylate-2,20-ipyridine) ruthenium(II)]at room temperature for 12 h. The platinized counter electrodes(Pt-TCO) were obtained by spraying H2PtCl6 solution to TCO glassfollowed by heating at 410 �C for 20 min. Then, the counter elec-trode was placed directly on the top of the dyed TiO2 film sealed

Freq

uenc

y /H

z

10-2

107

The series resistance

TCO/TiO2 interface

EL/Pt-TCO interface

TiO2/EL interface

The Nernst impedance

Z'

Z'

Z'

Z'

Z'

Z'Z'

Z''

Z''

Z''

Z''

Z''

Z''

Z''

a

b c

Figure 2. (a) Four semi-circles showing the kinetics of electrochemical process atthe TCO/TiO2 interface, the TiO2/EL interface, EL/Pt-TCO interface, and the Nernstdiffusion impedance respectively are observed gradually with the increase offrequency in an EIS Nyquist plot; (b) EIS Nyquist plot of a DSC contains four semi-circles; (c) EIS Nyquist plot of a DSC contains three semi-circles.

146 W. Liu et al. / Chemical Physics Letters 513 (2011) 145–148

with thermal adhesive films. And the electrolyte was filled from ahole made on the counter electrode, which was later sealed by acover glass and thermal adhesive films.

The method for electropolymerization of PPO (poly (phenyleneoxide-co-2-allylphenylene oxide)) was adapted from a literatureprocedure [17]. EIS are measured with an electrochemical worksta-tion (IM6ex, Zahner Corp., Germany). The frequency range wasexplored from 9.6 � 105–5.0 � 10�2 Hz and the ac amplitude was5 mV.

3. Results and discussion

Since the TiO2 film cannot completely cover the TCO substrate,exposed TCO also contacts the electrolyte. The TCO/EL interface is aroute for the electron transfer to redox ions in the electrolyte. Theexposed TCO is covered by a thin layer of insulating polymer inorder to prevent electron transfer to redox ions via the TCO/ELinterface and simplifies the TCO/TiO2 + EL interface.

A built-in field du is present at the TCO/TiO2 interface in thedark caused by the difference between the work function of theTCO (WTCO) and the redox potential (Eredox) (Figure 1a) [8]. Whenthe PPO was electropolymerized on the exposed TCO, the electro-lyte did not directly contact the TCO and the TCO/EL interface isomitted [6]. The energy band diagram for DSC is showed in Figure1c and d after PPO deposition. There is a conduction bond discon-tinuity caused by the difference in TCO and TiO2 electron affinities,i.e. Dv ¼ vTCO � vTiO2

; (Figure 1c). When the applied potentialexceeds the flat band potential, a potential barrier is presented atthe TCO/TiO2 interface (Figure 1d). This situation is analogous tothe thermal emission model [8].

The process of electron transport and recombination in DSC canbe investigated with frequency-domain techniques, such as inten-sity modulated photocurrent spectroscopy (IMPS) [18,19], inten-sity modulated photovoltage spectroscopy (IMVS) [20,21] andelectrochemical impedance spectroscopy (EIS) [22–25]. EIS is apowerful technique to identify and study the kinetics of transportand recombination in DSC. EIS measures the current response to amodulated applied bias superimposed on a constant applied volt-age and EIS can be evaluated using resistance and capacitance ele-ments as an equivalent circuit [18–21,24,25]. In principle, the EISNyquist plot of a DSC contains four semi-circles (time constants)showing the kinetics of electrochemical process at the TCO/TiO2

interface, the TiO2/EL interface, EL/Pt-TCO interface, and the Nernstdiffusion impedance, respectively. Four semi-circles are observedgradually with the increase of frequency in an EIS Nyquist plot asillustrated in Figure 2a and b.

However, impedance of TCO/TiO2 interface overlaps with otherprocesses and it is hard to be directly observed [26,27]. Generally, atypical EIS Nyquist plot of a DSC only contains three semi-circles(Figure 2c). Thus the impedance of TCO/TiO2 interface in TiO2

Figure 1. Energy band diagrams of DSC before and after PPO deposition (

becomes immeasurable. The equivalent circuit of the complete cellis shown in Figure 3a. For simplicity, only the RC circuits areemployed here to model DSC at different interfaces. Rs is the seriesresistance. R0 and a capacitive element C0 are the charge transferresistance related to the recombination at the exposed TCO tothe electrolyte and the corresponding interface capacitance. R1

and C1 are the resistance and the interface capacitance at theTCO/TiO2 interface. R2 and C2 are the charge transfer resistanceand double layer capacitance at the EL/Pt-TCO interface. R3 andC3 are charge transfer resistance and chemical capacitance at theTiO2/EL interface. Ws is the Nernst diffusion impedance of redoxspecies contains three parameters: the dc resistance of the diffu-sion impedance (W–R), the characteristic diffusion time constant(W–T), the exponent (W–P = 0.5).

The equivalent circuits are employed here to model DSC beforeand after PPO deposition as indicated in Figure 3a and b. Since athin insulating layer was deposited to isolate the exposed TCOfrom the electrolyte, the equivalent circuit of the TCO/EL interfaceis negligible. The equivalent circuit of DSC after deposited PPO isshown in Figure 3b. Since electrons can only be transferred acrossthe TCO/TiO2 interface after PPO deposition, the semi-circle associ-ated with the impedance TCO/TiO2 interface was observed and foursemi-circles in a Nyquist plot were exhibited under a relativelynegative bias voltage. These are illustrated in Figure 3c showing

a) and (c) in the dark; (b) and (d) under illumination or forward bias.

40 60 80 100 120 1400

5

10

15

20

25

R 0

R 1

C 1 C 2 C 3

R 2 R 3 W sR s

C 0

R1

C1 C2 C3

R2 R3 WsRs

100000 1000 10 0.140

80

120 M ag Theta Fitting result

Frequency,Hz

Mag

0

-5

-10

-15

Theta

Z'',

Ohm

Z', Ohm

Data poit Fitting ressult

a

b

c

d

Figure 3. Equivalent circuit for (a) before and (b) after PPO deposition. (c) Nyquist plot and (d) Bode plots under forward bias (�0.75 V) in the dark.

W. Liu et al. / Chemical Physics Letters 513 (2011) 145–148 147

the Nyquist plot of a DSC after PPO deposition under forward bias(�0.75 V) in the dark. In the Bode plot, an extra characteristicfrequency appears in the region higher than the characteristic fre-quency of the EL/Pt-TCO interface (Figure 3d).

Table 1 presents the fitting results of the parameters of the DSCusing the equivalent circuit of Figure 3b. The fitting results showthat this electron transfer resistance is about 1.25 X cm2. The resis-tance of EL/Pt-TCO interface is the order of magnitude of about101 X cm2 and the resistance of TiO2/EL interface is the order ofmagnitude of about 101–102 X cm2. The resistance R1 at the TCO/TiO2 interface is far lower than the values result from otherinterfaces.

The current density J through the TCO/TiO2 interface is domi-nated by thermal emission of electrons from the TiO2 to TCOaccording to [28]:

J ¼ A�T2 exp�qUkBT

� �exp

qDVkBT� 1

� �ð1Þ

where U is Schottky barrier height, A⁄ is Richardson constant which inthis case is 6.71 � 106 A m�2 K�2 [29], kB is Boltzmann constant, T isabsolute temperatures, q is elementary charge, and DV is the lossvoltage at the TCO/TiO2 interface (which in this case is negative).

Table 1Kinetic parameters determined by fitting the electrochemical impedance spectra.

Resistance Rs R1 R2 R3 W–R39.6 X 5.0 X 14.7 X 46.5 X 19.0 X

Capacitance C1 C2 C3 W–P3.8 � 10�7 F 3.8 � 10�5 F 9.1 � 10�4 F 0.5

Time constant s1 s2 s3 W–T1.9 � 10�6 s 5.6 � 10�4 s 4.2 � 10�2 s 2.2 s

The resistance R is defined as @DV=@J with respect to the applied volt-age [30],

R ¼ kB

qA�Texp

qUkBT

� �exp

�qDVkBT

� �ð2Þ

In Eq. (2), when increasing the voltage loss and the rising barrierheight U on the TCO/TiO2 interface, the R increases quickly. Theflow of electrons through the TCO/TiO2 interface will cause somevoltage loss [14]. Smaller resistance indicates the actual voltageloss on this interface is small. For example, when the current flow-ing by is 20 mA cm�2 and the resistance is 1.25 X cm2, the voltageloss obtained by calculation is only 25 mV. The great majority ofthe external bias voltage does not drop on the TCO/TiO2 interface.When DV equals zero, we can derive the barrier height U � 0.56 eVfrom Eq. (2). This value is consistent with the value reported in therelevant reference [15].

The capacitances of EL/Pt-TCO and TiO2/EL interface are theorder of magnitude of about 10�5 and 10�3 F cm�2, respectively.C1 is about 1.52 � 10�6 F cm�2 and this value is also far lower thanthe values result from other interfaces.

Using the values of resistance and capacitance from EIS mea-sures, the time constants for charge transfer across the TCO/TiO2

interface, the TiO2/EL interface, EL/Pt-TCO interface are determinedby calculations, respectively. The product of R1 and C1 correspondsto the time constant for the charge transfer across the TCO/TiO2

interface after PPO deposition, and the time constant is generallythe order of magnitude of 10�6 s.

The electron transit time is associated with the electron trans-port from the injection sites to the substrate. Because this processis always affected by trapping/detrapping, the electron transport isevidently slow. The transit time is generally the order of magnitudeof 10�3 s [31], which is longer than the time constant for charge

148 W. Liu et al. / Chemical Physics Letters 513 (2011) 145–148

transfer across the TCO/TiO2 interface. The result indicates that thecollection of electrons on the substrate is limited by the electrontransport in the film.

4. Conclusions

In summary, an insulating film was electropolymerized on theexposed TCO to separate the electrolyte from the TCO/TiO2 + ELinterface which simplifies the complicated structure. EIS was usedto characterize the barrier height, the interfacial impedance andkinetic behavior of the TCO/TiO2 interface successfully for the firsttime. It suggests that the resistance at the TCO/TiO2 interface is farlower than the values result from other interfaces and the voltageloss on this interface is little. Kinetic studies indicate that the timeconstant for interface electron transfer across the TCO/TiO2 interfaceis about 10�6 s and the collection process of electrons on substrate islimited by the electron transport in the semiconductor film.

Acknowledgements

This work was financially supported by the National BasicResearch Program of China under Grant No. 2011CBA00700, theNational High Technology Research and Development Program ofChina under Grant No. 2009AA050603, China Postdoctoral ScienceFoundation Funded Project under Grant No. 20110490835, andFunds of the Chinese Academy of Sciences for Key Topics in Inno-vation Engineering under Grant No. KGCX2-YW-326.

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